Technical Notes - Helicopter Electromagnetics

The Basics of Kimberlite Geology

Kimberlite pipes are the primary source for diamonds in the world. These "carrot-shaped" intrusives (diatremes) are explosively intruded into older rocks by magma from the Earth's mantle, 100-150 km down. Because the kimberlites are often softer than the host rocks, they are usually weathered and buried by overburden and/or lakes. Geophysical surveys are essential tools for precisely locating these pipes.

Some of the important details in creating a diamond-bearing kimberlite are the depth from which the magma originates and the speed of the intrusion. It must come from deep enough that pressures and temperatures are high enough for carbon to be stable in the diamond crystal structure. The "explosive intrusion" is also critical. If the intrusive forms slowly, then there is time as the magma rises in the pipe and temperatures fall for the diamond to be re-absorbed into lower temperature forms of carbon, such as graphite. (The term "explosive" is measured in geologic time; it has been suggested that a pipe could be intruded from depth in as little as 10 hours.)

While kimberlite pipes were once thought to be common only in South Africa, they have been recently found in many geologic environments in the world. The pipes are generally fairly young, about the Jurassic to Cretaceous periods (180-65 million years ago). The top part of some of the Northwest Territories pipes were found to have pieces of burned wood and coal in them. Pipes have been found intruded into host rocks of roughly the same age (Alberta), and in rocks of 2 billion years age (Lac de Gras, Northwest Territories). In Canada, pipes have also be found in the Kirkland Lake area, and in northern Ontario in the James Bay Lowlands.

There are a lot of studies on-going into types of kimberlite, many of the details of which would only be interesting to a geochemist, were it not for the fact that the differences may define which type of kimberlite is more likely to carry diamonds. The diameter of the pipes varies between 50m and 500m or more.

Our company concentrates on finding kimberlites using airborne geophysics (as a contract service to mining companies). You can find a lot of information on the geophysics of kimberlites on our website. Review that and then get back to me if you would like more information. Pipes are most often detected geophysically by their magnetic anomaly (they have above average amounts of iron), by their electrical conductivity (they are more conductive than most rocks) and by their gravitational attraction (they are more dense than average rocks).

The most common non-geophysical method used to find pipes is geological/geochemical tracing. Geologists sample the fine grained gravels over large areas, looking for the minerals characteristic of kimberlites. These minerals, such as garnets and chrome diopside, were gouged from the kimberlite and carried away by the continental glaciers into a "glacial dispersion train". (Non-microscopic diamonds are too rare in the kimberlite to give much chance of finding any far from the kimberlite. It's a lucky prospector who finds them.)

Once an area of promising indicator minerals has been found, prospecting and geophysical surveys are conducted to try and locate the kimberlite source. Because kimberlite is softer than many host rocks (such as the granites of the Lac de Gras area), they are usually deeply gouged by the glaciers, and form lakes which are often characteristically round. Airborne geophysics is the most efficient way to survey a large area in detail. Drilling is then conducted to confirm the presence of the pipe.

It is important to realise how scarce diamonds are, even in an economic kimberlite pipe. The pipes of the Ekati mine, which is currently Canada's only economic diamond mine, grade around 1 carat per tonne of kimberlite. The quality and size of these diamonds makes a big difference on the value of the mine.

Greg Hodges, Chief Geophysicist, 2000